The invention relates to the field of disk drive memory devices, and, more particularly to magnetic head positioning servo control units for disk drives.
In conventional disk drives the magnetic head assembly is positioned by determining the seek length, i.e., the number of magnetic tracks on the surface of the disk which must be crossed, and then allowing uncontrolled positive acceleration of a servo arm bearing a magnetic head during the first uncontrolled part of the seek motion. A position sensor on the magnetic head assembly generates a track position signal which is differentiated by an electronic circuit to estimate the actual velocity of the head.
When certain criteria of track position and velocity are reached, the conventional systems begin to slow the head assembly servo arm down. Ideally, the deceleration profile should result in a slow down of the head to exactly zero velocity exactly as the head assembly arrives over the desired track, i.e., no overshoot. A predetermined deceleration profile is used for the last part of the trip, but the profile relates position to velocity according to a parametric position versus velocity equation where time is eliminated as a variable. The estimated velocity computed from the position data is then compared to a desired velocity for that position and an error signal is generated.
The conventional design has several disadvantages. For one, the uncontrolled positive acceleration means that there is no precise schedule for the acceleration as a function of time. The amount of power dissipated as heat in a servo coil is a function of (1/T).sup.4 where T is time to complete the seek. If there is no precise time schedule for the positive acceleration portion more power is supplied to the servo coil than for controlled acceleration. This power translates to heat
Further, the calculation of velocity in these conventional units leads to velocity figures which can be quite inaccurate. Because of this inaccuracy, the head assemblies sometimes undershoot or overshoot their target tracks at the end of the deceleration profile, i.e., the actual velocity differs from the velocity calculated by the system which controls the deceleration profile. The result is that when the profile velocity is supposed to be zero and the head is supposed to be "on track", the head has either stopped short of the target or is still moving and moves past the target. These undershoots and overshoots take time to correct, which means that more power must be supplied to make the entire servo movement faster to make up for the sometimes inaccurate calculated velocity. That is more power must be supplied to insure that the entire movement is fast enough so that even the slowest units satisfy acceptable specification limits for access time. The extra power dissipated in these conventional systems results in more power dissipation as heat in the servo coil which is located in a sealed capsule. Because the capsules are sealed, excess heat dissipation in them is a serious problem which can hasten the demise of various components of the head disk assembly.
The capsule must be sealed so that flying dust and smoke particles do not interfere with the head assembly. The heads in disk drives "fly" only 10-20 microinches above the disk surface such that even a cigarette smoke particle can interfere with this flying.
Accordingly, a need has arisen for a head positioner system for a disk drive which utilizes controlled acceleration profiles during both the acceleration and deceleration portions of the trip to prevent excess heat dissipation in the servos. Further, there is a need for a positioning servo which precisely controls the time for a seek to be completed. Precise time control enables predictable seek times to be specified in unit specifications.